1. Field of the Invention
The present invention relates to a method of and an apparatus for forming a shaped metallic article or forging a metallic article, and more particularly to a method of and an apparatus for forming an elongate article such as a crankshaft, for example, from a tubular or rod-shaped blank.
2. Prior Art
The so-called RR forging process is widely known and used to manufacture crankshafts, for example. According to the RR forging process, a rod-shaped blank is axially compressed by upsetting, and portions of the upset blank, which will correspond to crankpins, are bent by compression in a direction normal to the axial direction of the blank.
Various forging apparatus employing the RR forging process are known in the art, as disclosed in Japanese Patent Publications Nos. 43-12995 and 49-24337, for example.
The known forging apparatus have a die assembly composed of separate upper and lower dies, which are clamped and positioned by complex clamping and positioning mechanisms.
After crankshafts are formed by the RR forging process, they may be bent for various reasons.
More specifically, after the blank has been upset and bent, dies fitted over the opposite ends of the blank and a punch pressing the blank downwardly are separated from the formed blank. At this time, the crankshaft may be bent if the dies and the pressing punch are not displaced in synchronism with each other. Stated otherwise, if the pressing punch is lifted out of intimate contact with an upper surface of the crankshaft while the dies are not sufficiently spaced from the crankshaft but small gaps are left between the opposite ends of the crankshaft and the dies, the pressing punch as it is lifted may cause the crankshaft to be also lifted, depending on the degree of intimate contact between the pressing punch and the crankshaft. When this happens, the opposite ends of the crankshaft impinge upon the dies, and are bent thereby.
When the blank is bent in the RR forging process, residual stresses may be left in the crankshaft. Depending on such residual stresses, the crankshaft may be bent after it has been completely removed from the dies or the punch.
An elongate blank may also be bent in the vicinity of its center when punches are held against the opposite ends of the blank and axially pressed to axially swage or squeeze the blank.
In order to forge the blank with ease, it has been customary to heat the blank in advance. When the formed crankshaft is cooled, however, the crankshaft shrinks and may be bent.
Heretofore, a bent crankshaft has been corrected out of the bent configuration by a correcting press. Use of the press however adds a pressing step to the forging process, and is not economical since the cost of the die of the press is required.
Another way to correct a bent crankshaft has been to use a simple press for pressing the crankshaft in a direction opposite to the direction in which the crankshaft is bent. This correcting method is not efficient because the direction in which the crankshaft is bent and the extent to which it is bent have to be measured manually by a skilled worker, and the bent crankshaft cannot be corrected in many directions.
Another known correcting method is a rolling process by which a rotating grinding wheel is pressed against the entire circumference of a bent formed article, and the article is corrected out of the bent shape while it is being ground by the grinding wheel. Such a rolling process is used to correct a bent engine valve, for example. Some crankshafts, right after they are formed, have burrs produced between upper and lower dies. Since burrs are attached on opposite sides of the crankshaft shank and the crankshaft shank is not of a true circular shape, but is somewhat deformed, it is impossible to correct the crankshaft according to the rolling process.
Certain crankshafts are hollow because of the need for use with an oil gallery or because a reduction in weight is desired.
When a hollow crankshaft is to be produced, it has to be formed from a pipe blank which has a relatively large wall thickness inasmuch as the produced crankshaft requires a considerable degree of mechanical strength in its crankpins and journals. However, commercially available hollow blanks or pipes are limited with respect to diameter and wall thickness, and may not be suitable to the manufacture of optimum crankshafts. Since it is difficult to obtain commercially available hollow blanks of large wall thickness, an additional process is usually required to forge hollow blanks in addition to the squeezing process or the RR forging process. Especially when a crankshaft is to be produced from a commercially available hollow blank, since the opposite end of the crankshaft has a smaller diameter than those of the crankpins and journals, the hollow blank has to be swaged or squeezed to a large degree in order to obtain a desired configuration.
If the diameter of the opposite ends of the crankshaft is very small as compared with those of the crankpins and journals thereof, then the squeezing ratio or the percentage of diameter reduction becomes excessively large, causing the formed crankshaft to crack or buckle.
While it is still possible to forge the opposite ends of the crankshaft in an increased number of steps including the squeezing, as described above, the counterweights of a crankshaft produced by the RR forging, which counterweights are of bent or curved configuration, cannot be forged in such an increased number of steps.